Current mode PWM boost circuit and feedback signal sensing method thereof

ABSTRACT

A feedback signal sensing method includes the steps of: providing a pulse width modulation (PWM) signal having a period; charging a capacitor by a current source during a pulse duration of the period, so as to form an equivalent slope compensation ramp signal; conducting an inductor current flowing from a boost inductor to flow through an equivalent resistor during the pulse duration of the period, so as to form an equivalent inductor current signal; and using a coupling characteristic of the capacitor together with the equivalent slope compensation ramp signal and the equivalent inductor current signal to form a feedback signal.

CROSS-REFERENCE TO RELATED U.S. APPLICATIONS

Not applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

NAMES OF PARTIES TO A JOINT RESEARCH AGREEMENT

Not applicable.

REFERENCE TO AN APPENDIX SUBMITTED ON COMPACT DISC

Not applicable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a current mode pulse width modulation(PWM) boost circuit and a feedback sensing method thereof. Moreparticularly, the present invention relates to a current mode PWM boostcircuit having functions of directly sensing an inductor current and aslope compensation ramp signal and a feedback sensing method thereof.

2. Description of Related Art Including Information Disclosed Under 37CFR 1.97 and 37 CFR 1.98.

FIG. 1 shows a conventional current mode pulse width modulation (PWM)boost circuit 10, which includes a boost circuit 11, a voltage dividingcircuit 19, an error amplifier 12, a comparator 13, an inductor currentgenerator 14, a slope compensation ramp generator 15, an oscillator 16,a pulse width generator 17, and a buffer 18. A voltage V_(IN) isincreased by the boost circuit 11 to generate a higher DC output voltageV_(OUT). The boost circuit 11 includes an input capacitor C1, a boostinductor L, a MOS transistor T, a rectifying diode D, and an outputcapacitor C2. The input capacitor C1 is used to filter out ripplevoltage from the voltage V_(IN). When the MOS transistor T is turned on,the rectifying diode D has a reverse bias. At this time, a current flowsthrough the boost inductor L forward, such that the voltage on the boostinductor L increases. However, the current does not flow through theboost inductor L in an instant, but increases linearly and forms anelectromagnetic field. At this time, when the MOS transistor T is turnedon, the output current is provided by the output capacitor C2 only. Whenthe MOS transistor T is turned off, the boost inductor L cannot storeenergy, so the electromagnetic field stored in the boost inductor L isreleased. Thus, the polarity of the voltage on the boost inductor L isinverted, such that the boost inductor L releases the stored energy tothe output capacitor C2, and a voltage at a terminal (i.e., the node N3)of the rectifying diode D that is connected to the boost inductor L ishigher than the voltage V_(IN). This energy provides a load current, andmeanwhile charges the output capacitor C2 again. The voltage dividingcircuit 19 includes two resistors R1 and R2, which are connected inseries. A divided voltage V_(F1) is acquired at a node N2 that connectsthe resistors R1 and R2, and is sent to the error amplifier 12 where thedivided voltage V_(F1) is compared with a reference voltage V_(REF) togenerate an error signal E₀. After that, the error signal E₀ is comparedwith a feedback signal V_(SUM) by the comparator 13. The output of thecomparator 13 (i.e., VF₂) and an oscillation signal S1 coming from theoscillator 16 are input into the pulse width generator 17 together. Adriving signal S_(DR) generated by the pulse width generator 17 passesthrough the buffer 18 to generate a gate control signal S_(G), so as toadjust the conductive time of the MOS transistor T (i.e., to adjust thepulse duration of the driving signal S_(DR)), and further to control theDC output voltage V_(OUT).

The inductor current generator 14 receives a voltage signal V_(SEN) fromthe node N3, and the voltage signal V_(SEN) is processed by avoltage-to-current transfer structure (e.g., a resistor or atransconductance amplifier) therein to generate an inductor currentI_(SEN) flowing through the boost inductor L. FIGS. 2( a)-2(c)illustrate different access points N31, N32, and N33 of the voltagesignal V_(SEN) in the conventional art. The method to capture thevoltage signal V_(SEN) of FIG. 2( a) is more accurate than the methodsof the other two figures, but consumes more power. The methods tocapture the voltage V_(SEN) of FIGS. 2( b) and 2(c) are lossless andbetter than the method of FIG. 2( a), but respectively have problems oflower accuracy and matching. Moreover, after the voltage signal V_(SEN)is processed by the inductor current generator 14, the inductor currentI_(SEN) may be easily distorted. Furthermore, the slope compensationramp generator 15 is directed to solving problems such as open-loopinstability, sub-harmonic oscillation, and noise sensitivity in currentmode converters when operating in continuous conduction mode with a dutycycle of the driving signal S_(DR) larger than 50%. The slopecompensation ramp generator 15 receives an oscillation signal S₂ fromthe oscillator 16, and the oscillation signal S₂ is processed by avoltage-to-current transfer structure (e.g., a transconductanceamplifier) therein to generate a slope compensation ramp signal I_(SLO).Similarly, after being used by the slope compensation ramp generator 15,the slope compensation ramp signal I_(SLO) may be easily distorted.Finally, the inductor current I_(SEN) and the slope compensation rampsignal I_(SLO) flow through a resistor Rf, and generate the feedbacksignal V_(SUM) at a node N1.

BRIEF SUMMARY OF THE INVENTION

One aspect of the present invention is to provide a current mode PWMboost circuit, which uses a feedback signal generating unit including acurrent source and a capacitor to directly measure an inductor currentand an equivalent slope compensation ramp signal inside the current modePWM boost circuit, so as to generate a feedback signal and to adjust aDC output voltage, thereby reducing the problem of signal distortionwhen measuring an inductor current and generating a slope compensationramp signal in the conventional art.

Another aspect of the present invention is to provide a feedback signalsensing method applicable to a current mode PWM boost circuit, whichdirectly measures an inductor current flowing through a boost inductorin the boost circuit and a equivalent slope compensation ramp signalformed according to a slope characteristic when a current source chargesa capacitor, so as to generate a feedback signal and adjust a DC outputvoltage.

Accordingly, the present invention discloses a current mode PWM boostcircuit, which includes a boost unit, a voltage dividing circuit, anerror amplifier, a comparator, a pulse width generator, and a feedbacksignal generating unit. The boost unit includes a boost inductor and aswitch, and the boost unit is configured to increase a voltage togenerate a DC output voltage. The voltage dividing circuit is configuredfor generating a divided voltage from the DC output voltage. The erroramplifier is configured to generate an error signal by comparing areference voltage with the divided voltage. The comparator is configuredto generate a first signal by comparing the error signal with a feedbacksignal. The pulse width generator is configured to receive the firstsignal and a second signal coming from an oscillator to generate a thirdsignal to control the switch. The feedback signal generating unit iscoupled to the boost unit to generate the feedback signal, wherein thefeedback signal includes an equivalent inductor current signal flowingthrough the boost inductor and an equivalent slope compensation rampsignal.

The present invention also discloses a feedback signal sensing methodapplicable to a current mode PWM boost circuit, which includes the stepsof: providing a PWM signal having a period; charging a capacitor by acurrent source during the a pulse duration of the period to form anequivalent slope compensation ramp signal; conducting an inductorcurrent flowing from a boost inductor to flow through an equivalentresistor during the pulse duration to form an equivalent inductorcurrent signal; and using a coupling characteristic of the capacitortogether with the equivalent slope compensation ramp signal and theequivalent inductor current signal to form a feedback signal. In anembodiment of the present invention, the feedback signal is acquired ata connection point of the current source and the capacitor.

The current mode PWM boost circuit of the present invention does not usethe voltage-to-current transfer structure, but directly measures theinductor current, and uses the feedback signal generating unit todirectly generate the equivalent slope compensation ramp signal.Therefore, compared with the conventional art, the present invention hasthe advantages of (1) reducing the distortion of the feedback signal;(2) having a favorable response speed because of the direct measurementand signal generation; and (3) eliminating the problem of open-loopinstability in the conventional art.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The invention will be described according to the appended drawings.

FIG. 1 is a schematic view illustrating a conventional current mode PWMboost circuit.

FIGS. 2( a)-2(c) are schematic views illustrating different accesspoints of the voltage signal in the conventional art.

FIG. 3 is a schematic view illustrating a current mode PWM boost circuitaccording to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 3 illustrates a current mode PWM boost circuit 20 according to anembodiment of the present invention, which includes a boost unit 21, avoltage dividing circuit 29, an error amplifier 22, a comparator 23, apulse width generator 27, a buffer 28, and a feedback signal generatingunit 24. The boost unit 21 includes a boost inductor L′, a MOStransistor T′, a rectifying diode D′ connected to a connection point P1of the boost inductor L′ and the MOS transistor T′, an input capacitorC3 for filtering out the ripple voltage from the voltage V_(IN), and anoutput capacitor C5 connected between the rectifying diode D′ and aground terminal, wherein the output capacitor C5 is used for generatinga DC output voltage V_(OUT). The voltage dividing circuit 29 uses the DCoutput voltage V_(OUT) to generate a divided voltage VF₃. The voltagedividing circuit 29 includes a first resistor R3 connected to therectifying diode D′ and a second resistor R4 connected between the firstresistor R3 and the ground terminal, wherein the divided voltage VF₃ isacquired at a node P3 of connecting the first resistor R3 and the secondresistor R4. The error amplifier 22 compares a reference voltage V_(REF)and the divided voltage VF₃ to generate an error signal E′₀. Thecomparator 23 compares the error signal E′₀ and a feedback signalV′_(SUM) to generate a signal VF₄. The pulse width generator 27 receivesthe signal VF₄ and a signal S_(OSC) coming from an oscillator 26 togenerate a signal S′_(DR) for controlling the MOS transistor T′. Thebuffer 28′ is optional, and is used to improve the driving capability ofthe signal S′_(DR), so as to form a gate control signal S′_(G) tocontrol the MOS transistor T′. The feedback signal generating unit 24 iscoupled to the boost unit 21, so as to generate the feedback signalV′_(SUM), in which the feedback signal V′_(SUM) includes an equivalentinductor current signal (not shown) passing through the boost inductorL′ and an equivalent slope compensation ramp signal (not shown). Thefeedback signal generating unit 24 includes a capacitor C4 and a currentsource I_(S), which is connected in series with the capacitor C4. Aterminal of the capacitor C4 is coupled to the connection point P1 ofthe boost inductor L′ and the MOS transistor T′, and the current sourceI_(S) is coupled to the other terminal of the capacitor C4.

The current mode PWM boost circuit 20 of the embodiment of FIG. 3 isdifferent from the current mode PWM boost circuit 10 in terms of themethod of generating the feedback signal V′_(SUM). The method forsensing the feedback signal V′_(SUM) of the present invention will beillustrated in detail below.

When the MOS transistor T′ is turned on, an inductor current I_(L)′generated by the voltage V_(IN) and passing through the boost inductorL′ flows to the ground terminal through the MOS transistor T′ that isturned on. A level V′_(SEN) at the node P1 generated by the inductorcurrent I_(L)′ is calculated according to the following formula (1):V′ _(SEN) =V _(IN) ×DT _(S) ×Rds/L  (1)

where, DT_(S) stands for a pulse duration of the fourth signal S′_(DR)(i.e., the conductive time of the MOS transistor T′), Rds stands for theresistance of the MOS transistor T′ when being turned on, and L standsfor the inductance of the boost inductor L′.

As the level V′_(SEN) includes information about the inductor current I_(L)′, the level V′_(SEN) is also referred to as an equivalent inductorcurrent signal, which is associated with the voltage V_(IN), the boostinductor L′, the resistance Rds of the MOS transistor T′ when beingturned on, and a duty cycle of the pulse width generator. Moreover, thecurrent source I_(S) charges the capacitor C4 when the MOS transistor T′is turned on, so a voltage difference V_(SLO) is established between thenodes P1 and P2. Such voltage difference is calculated according to thefollowing formula (2):V _(SLO) =I _(S) ×DT _(S) /C  (2)

where, DT_(S) stands for the pulse duration of the fourth signal S′_(DR)(i.e., the conductive time of the MOS transistor T′), and C stands forthe capacitance of the capacitor C4. As the voltage difference V_(SLO)includes information about the slope compensation ramp signal (i.e., theslope characteristic when the capacitor C4 is charged is similar to thethird signal S_(OSC) generated by the oscillator 26), the voltagedifference V_(SLO) is also referred to as an equivalent slopecompensation ramp signal, which is associated with the current sourceI_(S), the capacitor C4, and the duty cycle of the pulse width generator27. Therefore, according to the coupling characteristic of the capacitorC4, the feedback signal V′_(SUM) acquired at the node P2 is the sum ofthe equivalent inductor current signal and the equivalent slopecompensation ramp signal. In other words,

$\begin{matrix}\begin{matrix}{V_{SUM}^{\prime} = {V_{SEN}^{\prime} + V_{SLO}}} \\{= {{V_{I\; N} \times {DT}_{S} \times {{Rds}/L}} + {I_{S} \times {{DT}_{S}/C}}}} \\{= {\left( {{V_{I\; N} \times {{Rds}/L}} + {I_{S}/C}} \right) \times {DTS}}}\end{matrix} & (3)\end{matrix}$where, (V_(IN) × Rds/L+ I_(S)/C)× DTs in formula (3) has acharacteristic of fixed slope.

It is known from the above illustration that the current mode PWM boostcircuit and the feedback signal sensing method directly measure theinductor current in the current mode PWM boost circuit and convert theinductor current to an equivalent inductor current signal with afeedback signal generating unit including a current source and acapacitor, and meanwhile charge the capacitor with the current source todirectly generate an equivalent slope compensation ramp signal havingthe slope characteristic, so as to form a feedback signal directly atthe connection point of the current source and the capacitor. Therefore,when compared with the conventional art, the present invention has theadvantages of (1) reducing the distortion of the feedback signal; (2)having a favorable response speed; and (3) eliminating the problem ofopen-loop instability.

The above-described embodiments of the present invention are intended tobe illustrative only. Numerous alternative embodiments may be devised bypersons skilled in the art without departing from the scope of thefollowing claims.

1. A feedback signal sensing method, applicable to a current mode pulsewidth modulation (PWM) boost circuit, said method comprising the stepsof: providing a PWM signal having a period; charging a capacitor by acurrent source during a pulse duration of said period to form anequivalent slope compensation ramp signal; conducting an inductorcurrent flowing from a boost inductor to flow through an equivalentresistor during said pulse duration to form an equivalent inductorcurrent signal; and using a coupling characteristic of said capacitortogether with said equivalent slope compensation ramp signal and saidequivalent inductor current signal to form a feedback signal.
 2. Thefeedback signal sensing method of claim 1, wherein the PWM signal isconfigured to turn on a switch inside the current mode PWM boost circuitin the pulse duration.
 3. The feedback signal sensing method of claim 2,wherein the switch is a MOS transistor.
 4. The feedback signal sensingmethod of claim 1, wherein the equivalent slope compensation ramp signalis defined by the following formula:I_(S)×DT_(S)/C wherein I_(S) stands for a current of the current source,DT_(S) stands for the pulse duration, and C stands for the capacitanceof the capacitor.
 5. The feedback signal sensing method of claim 1,wherein the equivalent slope compensation ramp signal is a voltagedifference across two terminals of the capacitor.
 6. The feedback signalsensing method of claim 1, wherein the equivalent resistor is a MOStransistor in a conductive state.
 7. The feedback signal sensing methodof claim 1, wherein the equivalent inductor current signal is a voltagesignal, and is defined by the following formula:V_(IN)×DT_(S)×Rds/L wherein V_(IN) stands for a voltage of a voltagesource generating the inductor current, DT_(S) stands for the pulseduration, Rds stands for the resistance of the equivalent resistor, andL stands for the inductance of the boost inductor.
 8. The feedbacksignal sensing method of claim 1, wherein the feedback signal is the sumof the equivalent slope compensation ramp signal and the equivalentinductor current signal.
 9. The feedback signal sensing method of claim1, wherein the feedback signal is acquired at a connection point of thecurrent source and the capacitor.
 10. A current mode PWM circuitcomprising: a boost unit comprising a boost inductor and a switch, saidboost unit being configured to increase voltage to generate a DC outputvoltage; a voltage dividing circuit configured for generating a dividedvoltage from said DC output voltage; an error amplifier configured togenerate an error signal by comparing a reference voltage with saiddivided voltage; a comparator configured to generate a first signal bycomparing said error signal with a feedback signal; a pulse widthgenerator configured to receive said first signal and a second signal,said second signal coming from an oscillator to generate a third signal,wherein said third signal is configured to control said switch; said afeedback signal generating unit coupled to said boost unit to generatesaid feedback signal, wherein said feedback signal comprises anequivalent inductor current signal flowing through said boost inductorand an equivalent slope compensation ramp signal, said feedback signalgenerating unit comprising: a capacitor having a terminal coupled to aconnection point of said boost inductor and said switch; and a currentsource coupled to another terminal of said capacitor.
 11. The currentmode PWM boost circuit of claim 10, wherein said boost unit furthercomprises: a rectifying diode connected to said connection point of saidboost inductor and said switch; and an output capacitor connectedbetween said rectifying diode and a ground terminal to generate said DCoutput voltage.
 12. The current mode PWM boost circuit of claim 11,wherein said voltage dividing circuit comprises: a first resistorconnected to said rectifying diode; and a second resistor connectedbetween said rectifying diode and said ground terminal, wherein thedivided voltage is acquired at a connection point of said first resistorand said second resistor.
 13. The current mode PWM boost circuit ofclaim 10, further comprising a buffer connected to an output terminal ofsaid pulse width generator to improve the driving capability of saidthird signal so as to control said switch.
 14. The current mode PWMboost circuit of claim 10, wherein the equivalent slope compensationramp signal is generated when the current source charges said capacitorwith the switch turned on.
 15. The current mode PWM boost circuit ofclaim 10, wherein the equivalent slope compensation ramp signal isassociated with a current source, said capacitor, and a duty cycle ofsaid pulse width generator.
 16. The current mode PWM boost circuit ofclaim 10, wherein the equivalent inductor current signal is a voltagesignal and is associated with a resistance of said switch when beingturned on.
 17. The current mode PWM boost circuit of claim 10, whereinthe equivalent inductor current signal is a voltage signal and isassociated with the voltage, said boost inductor, a resistance of saidswitch when being turned on, and a duty cycle of said pulse widthgenerator.